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What Is the Process of Radiation? Master the Art | Diabatix

February 3, 2023
by
Ine Vandebeek

Radiation is the transfer of energy through electromagnetic waves, such as light and heat. In CFD (Computational Fluid Dynamics) simulations, radiation can significantly affect the overall heat transfer process. Therefore, it is important to consider whether or not to include radiation in your CFD simulations. And if so, at what stage of the simulation process.

There are several factors to consider when deciding whether or not to include radiation in your CFD simulations:

• Temperature: If the temperature of the fluid or solid surfaces in your simulation is high enough, radiation can become a significant mode of heat transfer. As a general rule of thumb, if the temperature of the surfaces is above approximately 500 K (227 °C), radiation should be considered.
• Emissivity: The emissivity of a surface is a measure of its ability to emit radiation. Surfaces with high emissivity, such as black bodies, emit more radiation than surfaces with low emissivity, such as shiny metallic surfaces. If the surfaces in your simulation have high emissivity, radiation is more likely to be an important mode of heat transfer and should be included in your simulation.
• Flow conditions: The flow conditions in your simulation can also affect the importance of radiation. For example, if the flow is turbulent or large temperature gradients are present, radiation is more likely to be important.

Several methods are available for modeling radiation if you decide to include radiation in your CFD simulation. In Coldstream, the FvDOM approach is used. This method is based on the Discrete Ordinate method and solves the Radiative Transfer Equation using a finite volume approach.

Let's take a look at two examples:

1. Imagine we are simulating the flow of air inside a furnace. The furnace walls are made of a transparent solid, such as glass, and the wall's temperature is above 500 K (227°C). We know that radiation should be included in our simulation based on the factors discussed earlier.
2. On the other hand, imagine we are simulating airflow inside a room with a comfortable temperature of 293 K (20 °C). The room's walls are made of a low-emissivity material, such as painted drywall. The temperature of the walls is below 500 K. Thus, based on the guidelines discussed earlier, we know that radiation is not likely to be a significant mode of heat transfer in this case and can be safely excluded from the simulation. In this specific case, conduction and convection will be the dominant modes of heat transfer between the fluid and the solid surfaces. By excluding radiation from the simulation, computation time and resources can be saved without sacrificing accuracy.

In conclusion, it is important to consider whether or not to include radiation in your CFD simulations, as it can play a significant role in the overall heat transfer process. Factors to keep in mind when deciding whether or not to include radiation include temperature, emissivity and flow conditions.